ON SALTS COLOURED BY CATHODE RAYS. 255 
attended by the emission of phosphorescent light. This is in accord- 
ance’ with the experience of Wiedemann and Schmidt that if the 
destruction of the colour is produced by heating, likewise a phos- 
phorescent light is produced, which in this case is strong but of a 
short duration, corresponding to the quick destruction of the after- 
colours by strong heating. 
If the salts, after having been coloured in the condition of a fine 
powder and then having been put between two glass plates (in order 
to obtain a plane surface), are placed in a photographic camera instead 
of the photographic plate, you may get a fine phosphorescent picture 
of a landscape or of architecture after a very short exposure. 
Time does not allow me to mention in detail several other 
peculiarities which are shown by matter in the distension state. In 
one direction only I may be allowed to make some remarks. 
The first-class after-colours may be produced not only by cathode 
rays but also by the B rays of radioactive substances, as you probably 
know. But they may also be produced by ultra-violet light, for 
instance, by ultra-violet spark light, even when a quartz plate is inter- 
posed between the spark and the salt. More than thirty years ago [ 
brought forward a hypothesis, according to which in every point where 
cathode rays strike a solid body a thin layer of ultra-violet light- 
radiating molecules is produced in the gas, to which ultra-violet light of 
very short wave-lengths, for instance, the phosphorescence of the glass 
walls in the cathode rays, is due. But I came further to the assumption 
that nearly all effects which are commonly ascribed to special qualities 
of the cathode rays, and likewise of § rays and x rays, are mere effects 
of the ultra-violet light which is produced by the stopping of these rays. 
I have been guided by this assumption during many years, and have 
very often been aided by it in foreseeing new phenomena. For 
instance, in this way I was induced to expect that the after-colours 
would be produced not only by cathode rays but also by the ordinary 
ultra-violet light; further I could guess that also the x rays would 
produce after-colours (which in this case have been observed by 
Holzknecht), and in recent times I could foresee that solid aromatic 
substances (the benzene derivatives) in the ultra-violet light must change 
their spectra of ordinary phosphorescence, composed of broad bands, 
and turn to peculiar spectra composed of narrow stripes, the wave- 
lengths of which are characteristic of the single aromatic substances.° 
So I believe also that the after-colours are produced not directly by the 
cathode rays or by B rays, but by the aforesaid ultra-violet light which 
is connected with the stopping of the other rays. 
In this way the after-colours enter at once into a great class of 
phenomena known as reversible effects of light. You know that certain 
effects of the visible spectral rays are destroyed by rays of longer 
wave-lengths, by the infra-red rays. And the analogy to this 
phenomenon is in my opinion the destruction of the after-colours: they 
are produced by the ultra-violet light of the stopped cathode rays and 
are annihilated by the longer visible wave-lengths of daylight. In this 
way you may likewise understand, for instance, that the coloured 
5 KE. Goldstein, Verhandl. d. D. Physik. Ges. 12. 
